Restrict list of available keys to encryption keys

[rlhawk]

When the user is presented with the list of available keys to use for encryption, it should be filtered so that only keys that match their encryption method are shown. As a first step, we can just filter by key type group ("encryption") and make further adjustments later to improve the behavior. Implementing this means that we will not be able to use the Key Select Form API element yet, since it's not currently possible to filter the key list by key type group.

[svendecabooter]

Recently committed changes filter the available keys based on the defined allowed key type(s) in the encryption method.

So for this issue to be tackled, I think we'd have to wait for the Key module's key_select element to add filtering based on group. I'll see if I can provide a patch to the Key module, but this seems fairly low priority to me, not blocking a first release of Encrypt module.

[svendecabooter]

See https://www.drupal.org/node/2678084 for followup